const banner = document.getElementsByClassName("banner")[0];
const blocks = document.getElementsByClassName("blocks");

for(var i=1;i<400;i++){
    banner.innerHTML += "<div class='blocks'></div>";
    const duration = Math.random()*5;
    blocks[i].style.animationDuration = duration +'s';
    blocks[i].style.animationDelay = duration +'s';
}
const section = document.querySelector("section");
setTimeout( function(){
    section.classList.add('active')
},14000);